1. Field of the Invention
The present invention relates generally to an active matrix display device, and more particularly to an active matrix display device in which switching elements such as thin film transistors apply drive signals to pixel electrodes arranged in a matrix so as to achieve a high density display.
2. Description of the Prior Art
In the known liquid crystal display devices, EL displays devices, plasma display devices, etc. the pixel electrodes arranged in a matrix are selectively driven so as to represent a pattern on a screen. The pixel electrodes are individually provided with switching elements through which the pixel electrodes are selectively driven. This is commonly called an active matrix driving system. The switching elements are made of a thin film transistor (TFT), a metal-insulator metal (MIM), a MOS transistor, a diode and a varistor. A voltage applied between the individual pixel electrodes and counter electrodes is switched on, and a liquid crystal, an EL light emitting medium, a plasma light emitting device make the display medium optically modulated. The optical modulation is observed as displayed patterns by the naked eye. The active matrix display device is suitable for display in high contrast, and finds application in liquid crystal television, word processors, and terminal display units of computers.
FIGS. 9 and 10 show known types of active matrix liquid crystal display devices. One of a pair of substrates has gate buses 21 arranged transversely and source buses 23 perpendicular to the gate buses Every rectangular segment enclosed by the adjacent gate buses 21 and the source buses 23 have pixel electrodes 41.
A gate bus branch 22 branched off from the gate bus 21 has a TFT 31 as a switching element. The gate bus branch 22 includes a first section which functions as a gate electrode for the TFT 31, and a second section which is narrower than the first section. A drain electrode 33 of the TFT 31 is electrically connected to the pixel electrodes 41, and a source electrode 32 is connected to the source bus 23.
FIG. 10 shows another known example in which a source bus branch 90 branched from the source bus 23 overlaps the gate bus 21, and a TFT 31 is formed on the overlapping part. A drain electrode 33 of the TFT 31 is electrically connected to the pixel electrodes 41, and a source electrode 32 is electrically connected to the source bus 23 through the source bus branch 90.
Under this arrangement of the known active matrix display device a problem arises, for example, if any switching element malfunctions, the pixel electrodes connected thereto receives no signal. This appears as devoid of a pixel electrode. The absence of a pixel electrode spoils the representation of the display device. This results in a lower manufacturing yield.
Such faulty or defective pixel electrodes occurs for the following two reasons:
(1) The pixel electrodes are not fully charged until the switching elements are selected by a scanning signal (signals from the gate bus) (hereinafter referred to as "on fault"), and
(2) An electric leakage occurs through the charged pixel electrodes before the switching elements are selected (hereinafter referred to "off fault").
The "on fault" occurs owing to a defective switching element. The "off fault" occurs for a further two reasons; one is an electrical leakage through the switching element, and the other is an electrical leakage between the pixel electrodes and the buses. In either case, the voltage to be applied between the pixel electrodes and the counter electrode does not reach a required value. This causes faulty pixel electrodes to look like luminous points under the normal white mode (a mode in which the optical transmissibility reaches the maximum when the voltage applied to the liquid crystal is zero), and looks like a black point under the normal black mode (a mode in which the transmissibility is lowest when the voltage reaches zero).
If these faults are found during the fabrication of a substrate in which the switching elements are arranged, they can be trimmed by laser. In fact, however, it is almost impossible to find a single faulty one in a great number of pixel electrodes. The mass production of substrates cannot be carried out without increasing costs and prolonging the time. It is completely impossible to do so in a large size display panel having 100,000 to 500,000 pixel electrodes.
It is possible to visually observe a faulty pixel electrode by overlaying a counter substrate on a substrate in question, injecting liquid crystal therebetween and applying an inspection signal to the source bus. This method requires a correction in which the source bus and the pixel electrodes are short-circuited so as to effect the charge and discharge of the pixel electrodes by a signal voltage irrespective of whether the source bus was selected or not. In the example shown in FIGS. 9 and 10, it is inherently difficult to do such a correction because of the arrangement of the source bus 23 and the pixel electrodes 41. After all, the display device containing the faulty pixel electrodes has to be discarded even if other components and elements are in good condition. This is wasteful, and increases the production cost. This accounts for the decreased manufacturing yield.